Field of the Invention
The invention relates to a non-grain-oriented electrical steel strip or sheet for electrotechnical applications, to an electrotechnical component manufactured from such an electrical steel strip or sheet and to a method for producing an electrical steel strip or sheet.
Description of Related Art
Non-grain-oriented electrical steel strips or sheets, also referred to in the industry as “NGO electrical steel strips or sheets”, are used to strengthen the magnetic flux in iron cores of rotating electrical machines. Such sheets are typically used for electric motors and generators.
In order to increase the efficiency of such machines, the highest rotational speeds or largest diameters possible are sought for the components respectively rotating in operation. As a result of this trend, the electrically relevant components manufactured from electrical steel strips or sheets of the type in question here are exposed to a high mechanical load which often cannot be met by the types of NGO electrical steel strip available today.
An NGO electrical steel strip or sheet is known from U.S. Pat. No. 5,084,112, which has a yield point of at least 60 kg-f/mm2 (approx. 589 MPa) and is manufactured from a steel which, in addition to iron and unavoidable impurities, contains (in wt. %) up to 0.04% C, 2.0%—less than 4.0% Si, up to 2.0 Al, up to 0.2% P and at least one element from the group “Mn, Ni”, wherein the total contents of Mn and Ni is at least 0.3% and at most 10%.
In order to obtain an increase in strength by the formation of carbon nitrides, the steel known from U.S. Pat. No. 5,084,112 contains at least one element from the group “Ti, V, Nb, Zr”, wherein in the case of the presence of Ti or V the Ti content % Ti and the V content % V in relation to the C content % C and the respectively unavoidable N content % N of the steel should satisfy the condition [0.4×(% Ti+% V)]/[4×(% C+% N)]<4.0. A strength-increasing effect is also attributed to the presence of phosphorus in the steel. However, the presence of higher contents of phosphorus is advised against, since it can cause grain boundary brittleness. In order to counteract this problem, which is considered to be serious, an additional B content of 0.001-0.007% is proposed.
The steel composed in such a way is cast into slabs according to U.S. Pat. No. 5,084,112, which are subsequently hot rolled into a hot strip which is optionally annealed, then pickled and after that cold rolled into a cold strip having a specific final thickness. The cold strip obtained is subsequently subjected to a recrystallising annealing process, in which it is annealed at an annealing temperature which is at least 650° C. but less than 900° C.
In the case of the presence of effective contents of Ti and P and B, N, C, Mn and Ni in the steel at the same time, although the NGO electrical steel strips or sheets produced according to U.S. Pat. No. 5,084,112 achieve yield points of at least 70.4 kg-f/mm2 (688 MPa), at the same time the hysteresis losses P1.5 are at least 6.94 W/kg with a sheet thickness of 0.5 mm and at a polarisation of 1.5 Tesla and a frequency of 50 Hz. Such high hysteresis losses are no longer acceptable for modern electrotechnical applications. Furthermore, in the case of many such applications, the hysteresis losses are of great importance at higher frequencies.